[C][Client] Check the pointer before deleting operation to avoid core dump

Created by: ityuhui

The c client code snippet that I generate using OpenAPI c generator ( master branch ) is below:

void v1_pod_spec_free(v1_pod_spec_t *v1_pod_spec) {
    v1_affinity_free(v1_pod_spec->affinity); <-- If v1_pod_spec is null, program will coredump here
...
}

void list_free(list_t *list) {
      list_iterateThroughListForward(list, listEntry_free, NULL);   <-- If list is null, program will coredump here.
      free(list);
}

So I add some guard, now the code becomes:

void v1_pod_spec_free(v1_pod_spec_t *v1_pod_spec) {
    if(NULL == v1_pod_spec){  <-- now if v1_pod_spec is null, the function will return.
       return ;
    }    
    v1_affinity_free(v1_pod_spec->affinity);
...
}

void list_free(list_t *list) {
      if(list){   <-- now if list is null, the function will bypass this code.         
          list_iterateThroughListForward(list, listEntry_free, NULL);
          free(list);
      }
}

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